Method of determining at least one parameter that is characteristic of the angular distribution of light illuminating an object in a projection exposure apparatus

ABSTRACT

A method of determining at least one parameter that is characteristic of the angular distribution of light illuminating an object in a projection exposure apparatus is described. This parameter may be, for example, a pupil asymmetry. The method comprises the step of inserting a filter element in or in close proximity of a pupil plane of an illumination system that is arranged between a light source and the object. The filter element has a filter function that varies in an azimuthal direction with respect to the optical axis of the illumination system. Then the intensity of the light in a plane downstream of the pupil plane is measured. After rotating the filter element around the optical axis by an angle Φ, the measurement of the intensity is repeated. From the filter function, the angle Φ and the measured intensities the parameter is measured.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of International ApplicationPCT/EP02/13430, with an international filing date of Nov. 28, 2002,whose contents is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method of determining at leastone parameter that is characteristic of the angular distribution oflight illuminating an object in a projection exposure apparatus.

[0004] 2. Description of Related Art

[0005] To achieve an optimum projection result in a projection exposureapparatus, it is important that all structural directions of an objectto be projected, in particular its horizontal and vertical structures,are imaged with optimum contrast. Such optimum contrast may not beachieved if the illumination of a pupil plane in the illuminating systemdeviates from its specified values, for example because it isasymmetrical and in particular astigmatic.

[0006] To be able to determine the projection quality of a projectionexposure apparatus, the determination of the degree of homogeneity ofthe illumination of the pupil plane of the illuminating system istherefore necessary. This is a parameter that is characteristic of theangular distribution of the light illuminating the object to beprojected.

[0007] According to a method known from commercially available productsit is known to determine this parameter in that the illuminationgeometry is changed by inserting apertures in the beam path of the lightsource. Such a method is time-consuming.

SUMMARY OF THE INVENTION

[0008] It is therefore an object of the present invention to simplifythe determination of the parameter that is characteristic of the angulardistribution of the light illuminating an object in a projectionexposure apparatus.

[0009] According to the invention, this object is achieved by a methodcomprising the following steps:

[0010] a) insertion of a filter element in or in close proximity of apupil plane of an illumination system arranged between a light sourceand the object, said filter element having a filter function that variesin an azimuthal direction with respect to the optical axis of theillumination system;

[0011] b) measurement of the intensity of the light in a planedownstream of the pupil plane;

[0012] c) rotation of the filter element around the optical axis by anangle Φ;

[0013] d) re-measurement of the intensity of the light in a planedownstream of the pupil plane,

[0014] e) calculation of the at least one parameter from the filterfunction, the angle Φ and the intensities measured in steps b) and d).

[0015] In the simplest embodiment, a single rotation step and twointegral intensity measurements are sufficient for the determination ofthe parameter. This method allows to determine the pupil asymmetry,which is characteristic of the angular distribution of the light and isa suitable quantity for determining deviations from an idealillumination. Such an ideal illumination may be, for example, adirectionally independent illumination.

[0016] The new method can be performed rapidly, which accelerates theadjustment of the projection exposure apparatus and thereby increasesits throughput.

[0017] The filter element may be rotated by different angles Φ, and thestep e) is then performed for each of these angles Φ.

[0018] Depending on the requirements imposed on the precision of thedetermination of the angular distribution of the light, the filterfunction of the filter element and the number of rotational steps can beadapted for the refined determination of the parameter.

[0019] This refined parameter measurement results in a precisedetermination of the angular distribution of the illumination, which isadvantageous for the projection of objects of complicated shape.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Various features and advantages of the present invention may bemore readily understood with reference to the following detaileddescription taken in conjunction with the accompanying drawing in which:

[0021]FIG. 1 shows a diagrammatic overview of a projection exposureapparatus for microlithography;

[0022]FIG. 2 shows a section, divided into quadrants, through theunobstructed aperture of an illuminating system of the projectionexposure apparatus in a pupil plane of the latter;

[0023]FIG. 3 shows a pupil filter disposed in the pupil plane of FIG. 2;

[0024]FIG. 4 shows a flowchart of a method of determining the pupilasymmetry of the illumination of the projection exposure apparatus; and

[0025]FIG. 5 shows diagrammatically the dependence of an intensity,which is measured in a wafer plane of the projection exposure apparatusin accordance with Figure 1, on the angle of rotation of the pupilfilter shown in FIG. 3 around the optical axis of the illuminatingsystem.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0026] A projection exposure apparatus denoted in its entirety by 1 inFIG. 1 and shown diagrammatically therein is used to transfer astructure of a mask 2 onto a wafer 3. The basic structure of anillumination system of the projection exposure apparatus 1 is describedin U.S. Pat. No. 6,285,443 which is incorporated therein by reference;therefore this illumination system is explained below with reference toFIG. 1 only to the extent necessary for the comprehension of the presentinvention.

[0027] Upstream of the illumination components in FIG. 1 a light source,not shown, for example a laser, is arranged that emits a projectionlight beam that is indicated in FIG. 1 by an arrow 4. An illuminationlens 5 is used to shape the projection light beam 4 for a first time.The illumination lens 5 comprises a multiplicity of optical components,of which two biconvex lenses 6, 7 are shown in FIG. 1 by way of example.

[0028] Disposed in a pupil plane of the illumination lens 5 is a pupilfilter 8 that is indicated in FIG. 1 by a broken line and that is linkedto an actuator 10 by means of a diagrammatically indicated rod 9. Thusthe pupil filter 8 can be rotated by a motor around the optical axis ofthe illumination lens 5, as is indicated by an arrow 11. Starting from apredetermined initial orientation of the pupil filter 8, the rotation ischaracterized by an angle of rotation Φ.

[0029] A parameter characterizing the imaging quality of theillumination lens 5 is the pupil asymmetry e inside the illuminationlens 5. To define said pupil asymmetry ε, the pupil plane of theillumination lens 5 that the projection light beam 4 passes through isdivided into four quadrants (cf. FIG. 2). In FIG. 2, the pupil plane isspanned by the Cartesian coordinate system of the projection exposureapparatus having the axes x, y. The pupil plane is divided intoquadrants in such a way that the quadrants are each halved into twoequally large sectors by the x- and y-axes, respectively. The twoquadrants H halved by the x-axis are described below as horizontalquadrants and the quadrants V halved by the y-axis as verticalquadrants. The pupil asymmetry e is defined as the ratio of theintensities of the components of the projection light beam 4 passingthrough the horizontal quadrants H and the vertical quadrants V. Thiscan be written as:

ε=I(H)/I(V)  (1)

[0030]FIG. 3 illustrates the filter function of the pupil filter 8. Forits part, the pupil filter 8 is divided into four quadrants, the twoupper and lower transmitting quadrants 12, 13 that are mutually oppositein FIG. 3 transmitting the components of the projection light beam 4that are incident upon them virtually completely, that is to say theyhave a transmission in the region of 100%. The two remaining right andleft attenuating quadrants 14, 15 that are mutually opposite in FIG. 3have a transmission in the region of 90% for the components of theprojection light beam 4 that are incident upon them. The pupil filter 8consequently has a filter function varying in the azimuthal directionwith respect to the optical axis of the illuminating system. Theattenuating quadrants may be designed as grey filter regions or aspartially reflecting regions.

[0031] After passing through the illumination lens 5, the projectionlight beam 4 is deflected through 90° by a flat deflecting mirror 16(cf. FIG. 1) and coupled into the left-hand end face of a glass-rodarrangement 17 by means of a coupling-in system not shown in FIG. 1. Theglass rod arrangement 17 is used to homogenize the projection light beam4, as is described, for example in U.S. Pat. No. 6,285,443 that has beenmentioned above. After passing through the glass-rod arrangement 17, theprojection light beam 4 enters a downstream lens 18 and is deflectedthrough 90° by a deflecting mirror 19 contained in the latter onto themask 2 in order to illuminate it.

[0032] A projecting lens 20, which likewise contains a multiplicity ofoptical components, of which two biconvex lenses 21, 22 arediagrammatically shown in FIG. 1, images the mask 2 on the wafer 3.

[0033] In the measurement configuration, shown in FIG. 1, of theprojection exposure apparatus 1, an intensity detector 25 is disposed inthe plane of the wafer 3 and can be displaced therein in two mutuallyperpendicular directions (cf. arrows 23, 24 in FIG. 1). Said intensitydetector is connected to a computer 27 via a signal line 26. Thedisplacement of the intensity detector 25 makes possible positionallyresolved measurements.

[0034] A method of determining the pupil asymmetry ε is now explainedwith reference to FIG. 4 as an example of a parameter characterizing theangular distribution of illumination of the projection light beam 4 inthe plane of the wafer 3:

[0035] First, in a preparatory step 28, the pupil filter 8 is insertedinto the illumination lens 5 and linked to the actuator 10 by means ofthe rod 9. The pupil filter 8 may also be permanently present in theillumination lens 5. In a first measurement position of the pupil filter8, in which the transmitting quadrants 12, 13 are aligned in such a waythat they completely cover the vertical quadrants V, the integralintensity of the projection light beam 4 is then measured in the planeof the wafer by means of the intensity detector 25. This takes place ina measurement step 29.

[0036] The integral intensity I₁ measured in the measurement step 29 canbe expressed as follows:

I ₁ =T(TQ)I(V)−T(AQ)I(H).  (2)

[0037] Here, I(V), I(H) are the intensity components, defined inconnection with formula (1), in the pupil plane of the illumination lens5. T(TQ) is the transmission of the transmitting quadrants 12, 13. T(AQ)is the transmission of the attenuating quadrants 14, 15.

[0038] In a rotation step 30, the pupil filter 8 is then rotated through90° around the optical axis of the illumination lens 5 by actuating theactuator 10. In this position, in a measurement step 31, the integralintensity of the projection light beam 4 is once more measured in theplane of the wafer with the aid of the intensity detector 25. Thissecond intensity, I₂, can be written as:

I ₂ =T(AQ)I(V)+T(TQ)I(H).  (3)

[0039] The pupil asymmetry ε is then calculated from the measured valuesI1, I2 in an evaluation/calculation step 32. This yields as intermediatevariables:

I ₂(H)=(T(AQ)I ₁ −T(TQ)I ₂)/(T(AQ)² −T(TQ)²)  (4)

[0040] and

I(V)=(T(AQ)I ₂ −T(TQ)I ₁)/(T(AQ)²)  (5)

[0041] Substitution in (1) yields the pupil asymmetry ε.

[0042] The pupil asymmetry ε is a direct measure of the angulardistribution of the projection light beam 4 in the plane of the wafer.

[0043] The method described above can be refined further by repeatedlymeasuring the integral intensity in the plane of the wafer at variousmeasurement positions of the pupil filter 8. For example, the pupilasymmetry ε may be determined not only in regard to a fixed laboratorycoordinate system (cf. xy coordinate system in FIG. 2), but that (ifany) coordinate system x′y′ rotated with respect to the fixed laboratorysystem may be determined in which the pupil asymmetry ε deviates themost from the value 1.

[0044] A method of doing this is explained below with reference to FIG.5. Steps in the method or components of the projection exposureapparatus that correspond to those that have already been described withreference to FIGS. 1 to 4 have reference numerals increased by 100 andare not explained in detail yet again.

[0045] In the method in accordance with FIG. 5, the pupil filter 108 isnot rotated through 90° in the rotation step 130, but is initiallyrotated to a first of a multiplicity of incremental measurementpositions that differ by a smaller angle of rotation, for example 10°.The pupil filter 108 is stopped at the individual measurement positionsand the integral intensity is measured in each measurement position byrepeatedly executing the steps 130 and 131, the integral intensitymeasured in every measurement position being temporarily stored in astorage step 135. This repeated execution is represented by the arrow133 in FIG. 5.

[0046] A result of such a measurement sequence is shown diagrammaticallyin FIG. 6. In the latter, the integral intensity I measured with theintensity detector 125 is shown as a function of the angle of rotation Φof the pupil filter 108. Because of the mirror symmetry of the filterfunction of the pupil filter 108, rotation in an angular range ofbetween 0 and 180° is sufficient. The continuous line in the I/Φ diagramof FIG. 6 is an idealized measurement result that is temporarily storedafter the storage steps 135.

[0047] The maximum I₁ of the measured I/Φ curve as well as its minimumI₂ as well as the associated angular positions Φ₁, Φ₂ are determined inthe course of an, in this case, expanded evaluation/calculation step132. Not only the maximum pupil asymmetry ε can be determined from theseextreme values as well as the associated angular positions, but also theangular position of the corresponding quadrants H′, V′ in the pupilplane having minimum and maximum transmitted intensity in the coordinatesystem x′, y′ defined by the angular positions Φ₁, Φ₂.

[0048] The division of the pupil filters 8, 108 into quadrants resultsin a simple determination of the pupil asymmetry ε, as shown above. Ifthe intensity distribution of the projection light beam 4 in the pupilplane of the illumination lens 5 is to be determined in greater detail,for example, using the measurement sequence explained within theframework of the discussion of FIGS. 5 and 6, the pupil filter 8, 108may also have another filter function. The transmitting regions (cf.transmitting quadrants 12, 13) may, for example, be designed astransmitting sectors having a sector angle differing from 90°, forexample a smaller sector angle or having a number of sectors differingfrom four. Further details of the intensity distribution of theprojection light beam 4 in the pupil plane of the illumination lens 5can be measured by a filter function of the pupil filter 8, 108 having aradial dependence of the transmission using appropriately adaptedalgorithms in the calculation step 32, 132.

[0049] It goes without saying that the transmission values in thetransmitting quadrants 12, 13, on the one hand, and the attenuatingquadrants 14, 15, on the other, may also assume values other than 100%or 90%, respectively. The decisive factor is that the transmissions inthe transmitting quadrants 12, 13, on the one hand, and the attenuationquadrants 14, 15, on the other, differ sufficiently for the parameterdetermination.

1. A method of determining at least one parameter that is characteristicof the angular distribution of light illuminating an object in aprojection exposure apparatus, said method comprising the followingsteps: a) insertion of a filter element in or in close proximity of apupil plane of an illumination system arranged between a light sourceand the object, said filter element having a filter function that variesin an azimuthal direction with respect to the optical axis of theillumination system; b) measurement of the intensity of the light in aplane downstream of the pupil plane; c) rotation of the filter elementaround the optical axis by an angle Φ; d) re-measurement of theintensity of the light in a plane downstream of the pupil plane, e)calculation of the at least one parameter from the filter function, theangle Φ and the intensities measured in steps b) and d).
 2. The methodof claim 1, wherein the filter element is rotated by different angles Φ,and wherein the step e) is performed for each of these angles Φ.